Add NetworkRequest.Builder creating from an existing instance

Provide a formal way to construct a new NetworkRequest from an
existing instance. The network capabilities inside the
NetworkRequest is hidden. There is no way to pass a
NetworkRequest and update its capabilities. Add
NetworkRequest.Builder creating from an existing instance to
allow to clone the network capabilities.

Bug: 172183305
Test: make update-api
Change-Id: I068462b2a1410daf67b0c95f2b643d396f079531
CTS-Coverage-Bug: 172183305